lm2596 simple switcher power converter 150 khz3a step …

LM2596SIMPLE SWITCHER® Power Converter 150 kHz3A Step-Down Voltage RegulatorGeneral DescriptionThe LM2596 series of regulators are monolithic integratedcircuits that provide all the active functions for a step-down(buck) switching regulator, capable of driving a 3A load withexcellent line and load regulation. These devices are avail-able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-able output version.

Requiring a minimum number of external components, theseregulators are simple to use and include internal frequencycompensation†, and a fixed-frequency oscillator.

The LM2596 series operates at a switching frequency of150 kHz thus allowing smaller sized filter components thanwhat would be needed with lower frequency switching regu-lators. Available in a standard 5-lead TO-220 package withseveral different lead bend options, and a 5-lead TO-263surface mount package.

A standard series of inductors are available from severaldifferent manufacturers optimized for use with the LM2596series. This feature greatly simplifies the design ofswitch-mode power supplies.

Other features include a guaranteed ±4% tolerance on out-put voltage under specified input voltage and output loadconditions, and ±15% on the oscillator frequency. Externalshutdown is included, featuring typically 80 µA standby cur-rent. Self protection features include a two stage frequencyreducing current limit for the output switch and an overtemperature shutdown for complete protection under faultconditions.

Featuresn 3.3V, 5V, 12V, and adjustable output versionsn Adjustable version output voltage range, 1.2V to 37V

±4% max over line and load conditionsn Available in TO-220 and TO-263 packagesn Guaranteed 3A output load currentn Input voltage range up to 40Vn Requires only 4 external componentsn Excellent line and load regulation specificationsn 150 kHz fixed frequency internal oscillatorn TTL shutdown capabilityn Low power standby mode, IQ typically 80 µAn High efficiencyn Uses readily available standard inductorsn Thermal shutdown and current limit protection

Applicationsn Simple high-efficiency step-down (buck) regulatorn On-card switching regulatorsn Positive to negative converterNote: †Patent Number 5,382,918.

Typical Application (Fixed Output VoltageVersions)

01258301

SIMPLE SWITCHER® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation.

May 2002LM

2596S

IMP

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WITC

HE

RP

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Converter

150kH

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Step-D

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VoltageR

egulator

© 2002 National Semiconductor Corporation DS012583 www.national.com

Connection Diagrams and Ordering InformationBent and Staggered Leads, Through Hole

Package5-Lead TO-220 (T)

Surface Mount Package5-Lead TO-263 (S)

01258302

Order Number LM2596T-3.3, LM2596T-5.0,LM2596T-12 or LM2596T-ADJ

See NS Package Number T05D

01258303

Order Number LM2596S-3.3, LM2596S-5.0,LM2596S-12 or LM2596S-ADJ

See NS Package Number TS5B

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.

Maximum Supply Voltage 45V

ON /OFF Pin Input Voltage −0.3 ≤ V ≤ +25V

Feedback Pin Voltage −0.3 ≤ V ≤+25V

Output Voltage to Ground

(Steady State) −1V

Power Dissipation Internally limited

Storage Temperature Range −65˚C to +150˚C

ESD Susceptibility

Human Body Model (Note 2) 2 kV

Lead Temperature

S Package

Vapor Phase (60 sec.) +215˚C

Infrared (10 sec.) +245˚C

T Package (Soldering, 10 sec.) +260˚C

Maximum Junction Temperature +150˚C

Operating ConditionsTemperature Range −40˚C ≤ TJ ≤ +125˚C

Supply Voltage 4.5V to 40V

LM2596-3.3Electrical CharacteristicsSpecifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range

Symbol Parameter Conditions

LM2596-3.3Units

(Limits)Typ

(Note 3)Limit

(Note 4)

SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1

VOUT Output Voltage 4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 3.3 V

3.168/3.135 V(min)

3.432/3.465 V(max)

η Efficiency VIN = 12V, ILOAD = 3A 73 %

LM2596-5.0Electrical CharacteristicsSpecifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range


LM2596-5.0Units

(Limits)Typ

(Note 3)Limit

(Note 4)


VOUT Output Voltage 7V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 5.0 V

4.800/4.750 V(min)

5.200/5.250 V(max)


LM2596-12Electrical CharacteristicsSpecifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range


LM2596-12Units

(Limits)Typ

(Note 3)Limit

(Note 4)


VOUT Output Voltage 15V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 12.0 V

11.52/11.40 V(min)

12.48/12.60 V(max)


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LM2596-ADJElectrical CharacteristicsSpecifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range


LM2596-ADJUnits

(Limits)Typ

(Note 3)Limit

(Note 4)


VFB Feedback Voltage 4.5V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 1.230 V

VOUT programmed for 3V. Circuit of Figure 1 1.193/1.180 V(min)

1.267/1.280 V(max)

η Efficiency VIN = 12V, VOUT = 3V, ILOAD = 3A 73 %

All Output Voltage VersionsElectrical CharacteristicsSpecifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range . Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V ver-sion. ILOAD = 500 mA


LM2596-XXUnits

(Limits)Typ

(Note 3)Limit

(Note 4)

DEVICE PARAMETERS

Ib Feedback Bias Current Adjustable Version Only, VFB = 1.3V 10 nA

50/100 nA (max)

fO Oscillator Frequency (Note 6) 150 kHz

127/110 kHz(min)

173/173 kHz(max)

VSAT Saturation Voltage IOUT = 3A (Notes 7, 8) 1.16 V

1.4/1.5 V(max)

DC Max Duty Cycle (ON) (Note 8) 100 %

Min Duty Cycle (OFF) (Note 9) 0

ICL Current Limit Peak Current (Notes 7, 8) 4.5 A

3.6/3.4 A(min)

6.9/7.5 A(max)

IL Output Leakage Current Output = 0V (Notes 7, 9) 50 µA(max)

Output = −1V (Note 10) 2 mA

30 mA(max)

IQ Quiescent Current (Note 9) 5 mA

10 mA(max)

ISTBY Standby Quiescent Current ON/OFF pin = 5V (OFF) (Note 10) 80 µA

200/250 µA(max)

θJC Thermal Resistance TO-220 or TO-263 Package, Junction to Case 2 ˚C/W

θJA TO-220 Package, Junction to Ambient (Note 11) 50 ˚C/W




ON/OFF CONTROL Test Circuit Figure 1

ON /OFF Pin Logic Input 1.3 V

VIH Threshold Voltage Low (Regulator ON) 0.6 V(max)

VIL High (Regulator OFF) 2.0 V(min)

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All Output Voltage VersionsElectrical Characteristics (Continued)Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-ture Range . Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V ver-sion. ILOAD = 500 mA


LM2596-XXUnits

(Limits)Typ

(Note 3)Limit

(Note 4)

IH ON /OFF Pin Input Current VLOGIC = 2.5V (Regulator OFF) 5 µA

15 µA(max)

IL VLOGIC = 0.5V (Regulator ON) 0.02 µA

5 µA(max)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device isintended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.

Note 3: Typical numbers are at 25˚C and represent the most likely norm.

Note 4: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used tocalculate Average Outgoing Quality Level (AOQL).

Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulatorsystem performance. When the LM2596 is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of ElectricalCharacteristics.

Note 6: The switching frequency is reduced when the second stage current limit is activated.

Note 7: No diode, inductor or capacitor connected to output pin.

Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.

Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistorswitch OFF.

Note 10: VIN = 40V.

Note 11: Junction to ambient thermal resistance (no external heat sink) for the TO-220 package mounted vertically, with the leads soldered to a printed circuit boardwith (1 oz.) copper area of approximately 1 in2.

Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single printed circuit board with 0.5 in2 of (1 oz.) copper area.

Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2 of (1 oz.) copper area.

Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2 of (1 oz.) copper area onthe LM2596S side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See Application Information in this data sheet and the thermalmodel in Switchers Made Simple™ version 4.3 software.

Typical Performance Characteristics (Circuit of Figure 1)

NormalizedOutput Voltage Line Regulation Efficiency

01258304 01258305 01258306

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Typical Performance Characteristics (Circuit of Figure 1) (Continued)

Switch SaturationVoltage Switch Current Limit Dropout Voltage

01258307 01258308 01258309

OperatingQuiescent Current

ShutdownQuiescent Current

Minimum OperatingSupply Voltage

01258310 01258311 01258312

ON /OFF ThresholdVoltage

ON /OFF PinCurrent (Sinking) Switching Frequency

01258313 01258314 01258315

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Typical Performance Characteristics (Circuit of Figure 1) (Continued)

Feedback PinBias Current

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Typical Performance CharacteristicsContinuous Mode Switching Waveforms

VIN = 20V, VOUT = 5V, ILOAD = 2AL = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ

Discontinuous Mode Switching WaveformsVIN = 20V, VOUT = 5V, ILOAD = 500 mA

L = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩ

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Horizontal Time Base: 2 µs/div.A: Output Pin Voltage, 10V/div.

B: Inductor Current 1A/div.

C: Output Ripple Voltage, 50 mV/div.

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Horizontal Time Base: 2 µs/div.A: Output Pin Voltage, 10V/div.

B: Inductor Current 0.5A/div.

C: Output Ripple Voltage, 100 mV/div.

Load Transient Response for Continuous ModeVIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A


Load Transient Response for Discontinuous ModeVIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A


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Horizontal Time Base: 100 µs/div.A: Output Voltage, 100 mV/div. (AC)

B: 500 mA to 2A Load Pulse

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Horizontal Time Base: 200 µs/div.A: Output Voltage, 100 mV/div. (AC)

B: 500 mA to 2A Load Pulse

Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

01258322

CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”

COUT — 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”

D1 — 5A, 40V Schottky Rectifier, 1N5825

L1 — 68 µH, L38

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Test Circuit and Layout Guidelines (Continued)

As in any switching regulator, layout is very important. Rap-idly switching currents associated with wiring inductance cangenerate voltage transients which can cause problems. Forminimal inductance and ground loops, the wires indicated byheavy lines should be wide printed circuit traces andshould be kept as short as possible. For best results,external components should be located as close to theswitcher lC as possible using ground plane construction orsingle point grounding.

If open core inductors are used, special care must betaken as to the location and positioning of this type of induc-tor. Allowing the inductor flux to intersect sensitive feedback,lC groundpath and COUT wiring can cause problems.

When using the adjustable version, special care must betaken as to the location of the feedback resistors and theassociated wiring. Physically locate both resistors near theIC, and route the wiring away from the inductor, especially anopen core type of inductor. (See application section for moreinformation.)

Adjustable Output Voltage Versions

01258323

where VREF = 1.23V

Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability.

CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”

COUT — 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”


L1 — 68 µH, L38

R1 — 1 kΩ, 1%

CFF — See Application Information Section

FIGURE 1. Standard Test Circuits and Layout Guides

LM2596

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LM2596 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

Given:

VOUT = Regulated Output Voltage (3.3V, 5V or 12V)

VIN(max) = Maximum DC Input Voltage

ILOAD(max) = Maximum Load Current

Given:

VOUT = 5V

VIN(max) = 12V

ILOAD(max) = 3A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from Fig-ures Figure 4, Figure 5, or Figure 6. (Output voltages of 3.3V,5V, or 12V respectively.) For all other voltages, see the designprocedure for the adjustable version.

B. From the inductor value selection guide, identify the induc-tance region intersected by the Maximum Input Voltage lineand the Maximum Load Current line. Each region is identifiedby an inductance value and an inductor code (LXX).

C. Select an appropriate inductor from the four manufacturer’spart numbers listed in Figure 8.


A. Use the inductor selection guide for the 5V version shownin Figure 5.

B. From the inductor value selection guide shown in Figure 5,the inductance region intersected by the 12V horizontal lineand the 3A vertical line is 33 µH, and the inductor code is L40.

C. The inductance value required is 33 µH. From the table inFigure 8, go to the L40 line and choose an inductor partnumber from any of the four manufacturers shown. (In mostinstance, both through hole and surface mount inductors areavailable.)

2. Output Capacitor Selection (C OUT)

A. In the majority of applications, low ESR (Equivalent SeriesResistance) electrolytic capacitors between 82 µF and 820 µFand low ESR solid tantalum capacitors between 10 µF and470 µF provide the best results. This capacitor should belocated close to the IC using short capacitor leads and shortcopper traces. Do not use capacitors larger than 820 µF .

For additional information, see section on output capaci-tors in application information section.

B. To simplify the capacitor selection procedure, refer to thequick design component selection table shown in Figure 2.This table contains different input voltages, output voltages,and load currents, and lists various inductors and output ca-pacitors that will provide the best design solutions.

C. The capacitor voltage rating for electrolytic capacitorsshould be at least 1.5 times greater than the output voltage,and often much higher voltage ratings are needed to satisfythe low ESR requirements for low output ripple voltage.

D. For computer aided design software, see Switchers MadeSimple™ version 4.3 or later.


A. See section on output capacitors in application infor-mation section.

B. From the quick design component selection table shown inFigure 2, locate the 5V output voltage section. In the loadcurrent column, choose the load current line that is closest tothe current needed in your application, for this example, usethe 3A line. In the maximum input voltage column, select theline that covers the input voltage needed in your application, inthis example, use the 15V line. Continuing on this line arerecommended inductors and capacitors that will provide thebest overall performance.

The capacitor list contains both through hole electrolytic andsurface mount tantalum capacitors from four different capaci-tor manufacturers. It is recommended that both the manufac-turers and the manufacturer’s series that are listed in the tablebe used.

In this example aluminum electrolytic capacitors from severaldifferent manufacturers are available with the range of ESRnumbers needed.

330 µF 35V Panasonic HFQ Series

330 µF 35V Nichicon PL Series

C. For a 5V output, a capacitor voltage rating at least 7.5V ormore is needed. But even a low ESR, switching grade, 220 µF10V aluminum electrolytic capacitor would exhibit approxi-mately 225 mΩ of ESR (see the curve in Figure 14 for the ESRvs voltage rating). This amount of ESR would result in rela-tively high output ripple voltage. To reduce the ripple to 1% ofthe output voltage, or less, a capacitor with a higher value orwith a higher voltage rating (lower ESR) should be selected. A16V or 25V capacitor will reduce the ripple voltage by approxi-mately half.

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LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 timesgreater than the maximum load current. Also, if the powersupply design must withstand a continuous output short, thediode should have a current rating equal to the maximumcurrent limit of the LM2596. The most stressful condition forthis diode is an overload or shorted output condition.

B. The reverse voltage rating of the diode should be at least1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) andmust be located close to the LM2596 using short leads andshort printed circuit traces. Because of their fast switchingspeed and low forward voltage drop, Schottky diodes providethe best performance and efficiency, and should be the firstchoice, especially in low output voltage applications. Ultra-fastrecovery, or High-Efficiency rectifiers also provide good re-sults. Ultra-fast recovery diodes typically have reverse recov-ery times of 50 ns or less. Rectifiers such as the 1N5400series are much too slow and should not be used.


A. Refer to the table shown in Figure 11. In this example, a 5A,20V, 1N5823 Schottky diode will provide the best perfor-mance, and will not be overstressed even for a shorted output.

4. Input Capacitor (C IN)

A low ESR aluminum or tantalum bypass capacitor is neededbetween the input pin and ground pin to prevent large voltagetransients from appearing at the input. This capacitor shouldbe located close to the IC using short leads. In addition, theRMS current rating of the input capacitor should be selected tobe at least 1⁄2 the DC load current. The capacitor manufactur-ers data sheet must be checked to assure that this currentrating is not exceeded. The curve shown in Figure 13 showstypical RMS current ratings for several different aluminumelectrolytic capacitor values.

For an aluminum electrolytic, the capacitor voltage ratingshould be approximately 1.5 times the maximum input volt-age. Caution must be exercised if solid tantalum capacitorsare used (see Application Information on input capacitor). Thetantalum capacitor voltage rating should be 2 times the maxi-mum input voltage and it is recommended that they be surgecurrent tested by the manufacturer.

Use caution when using ceramic capacitors for input bypass-ing, because it may cause severe ringing at the VIN pin.

For additional information, see section on input capaci-tors in Application Information section.


The important parameters for the Input capacitor are the inputvoltage rating and the RMS current rating. With a nominalinput voltage of 12V, an aluminum electrolytic capacitor with avoltage rating greater than 18V (1.5 x VIN) would be needed.The next higher capacitor voltage rating is 25V.

The RMS current rating requirement for the input capacitor ina buck regulator is approximately 1⁄2 the DC load current. Inthis example, with a 3A load, a capacitor with a RMS currentrating of at least 1.5A is needed. The curves shown in Figure13 can be used to select an appropriate input capacitor. Fromthe curves, locate the 35V line and note which capacitorvalues have RMS current ratings greater than 1.5A. A 680 µF/35V capacitor could be used.

For a through hole design, a 680 µF/35V electrolytic capacitor(Panasonic HFQ series or Nichicon PL series or equivalent)would be adequate. other types or other manufacturers ca-pacitors can be used provided the RMS ripple current ratingsare adequate.

For surface mount designs, solid tantalum capacitors can beused, but caution must be exercised with regard to the capaci-tor surge current rating (see Application Information on inputcapacitors in this data sheet). The TPS series available fromAVX, and the 593D series from Sprague are both surge cur-rent tested.

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LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

Conditions Inductor Output Capacitor

Through Hole Electrolytic Surface Mount Tantalum

Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague

Voltage Current Voltage (µH) ( #) HFQ Series PL Series Series 595D Series

(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)

3.3 3 5 22 L41 470/25 560/16 330/6.3 390/6.3

7 22 L41 560/35 560/35 330/6.3 390/6.3

10 22 L41 680/35 680/35 330/6.3 390/6.3

40 33 L40 560/35 470/35 330/6.3 390/6.3

6 22 L33 470/25 470/35 330/6.3 390/6.3

2 10 33 L32 330/35 330/35 330/6.3 390/6.3

40 47 L39 330/35 270/50 220/10 330/10

5 3 8 22 L41 470/25 560/16 220/10 330/10

10 22 L41 560/25 560/25 220/10 330/10

15 33 L40 330/35 330/35 220/10 330/10

40 47 L39 330/35 270/35 220/10 330/10

9 22 L33 470/25 560/16 220/10 330/10

2 20 68 L38 180/35 180/35 100/10 270/10

40 68 L38 180/35 180/35 100/10 270/10

12 3 15 22 L41 470/25 470/25 100/16 180/16

18 33 L40 330/25 330/25 100/16 180/16

30 68 L44 180/25 180/25 100/16 120/20

40 68 L44 180/35 180/35 100/16 120/20

15 33 L32 330/25 330/25 100/16 180/16

2 20 68 L38 180/25 180/25 100/16 120/20

40 150 L42 82/25 82/25 68/20 68/25

FIGURE 2. LM2596 Fixed Voltage Quick Design Component Selection Table

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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)

Given:

VOUT = Regulated Output Voltage

VIN(max) = Maximum Input Voltage

ILOAD(max) = Maximum Load Current

F = Switching Frequency (Fixed at a nominal 150 kHz).

Given:

VOUT = 20V

VIN(max) = 28V

ILOAD(max) = 3A

F = Switching Frequency (Fixed at a nominal 150 kHz).

1. Programming Output Voltage (Selecting R1 and R2, asshown in Figure 1 )

Use the following formula to select the appropriate resistorvalues.

Select a value for R1 between 240Ω and 1.5 kΩ. The lowerresistor values minimize noise pickup in the sensitive feed-back pin. (For the lowest temperature coefficient and the beststability with time, use 1% metal film resistors.)

1. Programming Output Voltage (Selecting R1 and R2, asshown in Figure 1 )

Select R1 to be 1 kΩ, 1%. Solve for R2.

R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.

R2 = 15.4 kΩ.


A. Calculate the inductor Volt • microsecond constant E • T (V• µs), from the following formula:

where VSAT = internal switch saturation voltage = 1.16V

and VD = diode forward voltage drop = 0.5V

B. Use the E • T value from the previous formula and match itwith the E • T number on the vertical axis of the Inductor ValueSelection Guide shown in Figure 7.

C. on the horizontal axis, select the maximum load current.

D. Identify the inductance region intersected by the E • T valueand the Maximum Load Current value. Each region is identi-fied by an inductance value and an inductor code (LXX).

E. Select an appropriate inductor from the four manufacturer’spart numbers listed in Figure 8.


A. Calculate the inductor Volt • microsecond constant

(E • T),

B. E • T = 34.2 (V • µs)

C. ILOAD(max) = 3A

D. From the inductor value selection guide shown in Figure 7,the inductance region intersected by the 34 (V • µs) horizontalline and the 3A vertical line is 47 µH, and the inductor code isL39.

E. From the table in Figure 8, locate line L39, and select aninductor part number from the list of manufacturers part num-bers.

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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)(Continued)



A. In the majority of applications, low ESR electrolytic or solidtantalum capacitors between 82 µF and 820 µF provide thebest results. This capacitor should be located close to the ICusing short capacitor leads and short copper traces. Do notuse capacitors larger than 820 µF. For additional informa-tion, see section on output capacitors in application in-formation section.

B. To simplify the capacitor selection procedure, refer to thequick design table shown in Figure 3. This table containsdifferent output voltages, and lists various output capacitorsthat will provide the best design solutions.

C. The capacitor voltage rating should be at least 1.5 timesgreater than the output voltage, and often much higher voltageratings are needed to satisfy the low ESR requirementsneeded for low output ripple voltage.

3. Output Capacitor SeIection (C OUT)

A. See section on COUT in Application Information section.

B. From the quick design table shown in Figure 3, locate theoutput voltage column. From that column, locate the outputvoltage closest to the output voltage in your application. In thisexample, select the 24V line. Under the output capacitor sec-tion, select a capacitor from the list of through hole electrolyticor surface mount tantalum types from four different capacitormanufacturers. It is recommended that both the manufactur-ers and the manufacturers series that are listed in the table beused.

In this example, through hole aluminum electrolytic capacitorsfrom several different manufacturers are available.

220 µF/35V Panasonic HFQ Series

150 µF/35V Nichicon PL Series

C. For a 20V output, a capacitor rating of at least 30V or moreis needed. In this example, either a 35V or 50V capacitorwould work. A 35V rating was chosen, although a 50V ratingcould also be used if a lower output ripple voltage is needed.

Other manufacturers or other types of capacitors may also beused, provided the capacitor specifications (especially the 100kHz ESR) closely match the types listed in the table. Refer tothe capacitor manufacturers data sheet for this information.

4. Feedforward Capacitor (C FF) (See Figure 1)

For output voltages greater than approximately 10V, an addi-tional capacitor is required. The compensation capacitor istypically between 100 pF and 33 nF, and is wired in parallelwith the output voltage setting resistor, R2. It provides addi-tional stability for high output voltages, low input-output volt-ages, and/or very low ESR output capacitors, such as solidtantalum capacitors.

This capacitor type can be ceramic, plastic, silver mica, etc.(Because of the unstable characteristics of ceramic capacitorsmade with Z5U material, they are not recommended.)

4. Feedforward Capacitor (C FF)

The table shown in Figure 3 contains feed forward capacitorvalues for various output voltages. In this example, a 560 pFcapacitor is needed.

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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)(Continued)



A. The catch diode current rating must be at least 1.3 timesgreater than the maximum load current. Also, if the powersupply design must withstand a continuous output short, thediode should have a current rating equal to the maximumcurrent limit of the LM2596. The most stressful condition forthis diode is an overload or shorted output condition.

B. The reverse voltage rating of the diode should be at least1.25 times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) andmust be located close to the LM2596 using short leads andshort printed circuit traces. Because of their fast switchingspeed and low forward voltage drop, Schottky diodes providethe best performance and efficiency, and should be the firstchoice, especially in low output voltage applications. Ultra-fastrecovery, or High-Efficiency rectifiers are also a good choice,but some types with an abrupt turn-off characteristic maycause instability or EMl problems. Ultra-fast recovery diodestypically have reverse recovery times of 50 ns or less. Recti-fiers such as the 1N4001 series are much too slow and shouldnot be used.


A. Refer to the table shown in Figure 11. Schottky diodesprovide the best performance, and in this example a 5A, 40V,1N5825 Schottky diode would be a good choice. The 5A dioderating is more than adequate and will not be overstressedeven for a shorted output.


A low ESR aluminum or tantalum bypass capacitor is neededbetween the input pin and ground to prevent large voltagetransients from appearing at the input. In addition, the RMScurrent rating of the input capacitor should be selected to be atleast 1⁄2 the DC load current. The capacitor manufacturersdata sheet must be checked to assure that this current ratingis not exceeded. The curve shown in Figure 13 shows typicalRMS current ratings for several different aluminum electrolyticcapacitor values.

This capacitor should be located close to the IC using shortleads and the voltage rating should be approximately 1.5times the maximum input voltage.

If solid tantalum input capacitors are used, it is recomendedthat they be surge current tested by the manufacturer.

Use caution when using a high dielectric constant ceramiccapacitor for input bypassing, because it may cause severeringing at the VIN pin.

For additional information, see section on input capaci-tors in application information section.


The important parameters for the Input capacitor are the inputvoltage rating and the RMS current rating. With a nominalinput voltage of 28V, an aluminum electrolytic aluminum elec-trolytic capacitor with a voltage rating greater than 42V (1.5 xVIN) would be needed. Since the the next higher capacitorvoltage rating is 50V, a 50V capacitor should be used. Thecapacitor voltage rating of (1.5 x VIN) is a conservative guide-line, and can be modified somewhat if desired.

The RMS current rating requirement for the input capacitor ofa buck regulator is approximately 1⁄2 the DC load current. Inthis example, with a 3A load, a capacitor with a RMS currentrating of at least 1.5A is needed.

The curves shown in Figure 13 can be used to select anappropriate input capacitor. From the curves, locate the 50Vline and note which capacitor values have RMS current ratingsgreater than 1.5A. Either a 470 µF or 680 µF, 50V capacitorcould be used.

For a through hole design, a 680 µF/50V electrolytic capacitor(Panasonic HFQ series or Nichicon PL series or equivalent)would be adequate. Other types or other manufacturers ca-pacitors can be used provided the RMS ripple current ratingsare adequate.

For surface mount designs, solid tantalum capacitors can beused, but caution must be exercised with regard to the capaci-tor surge current rting (see Application Information or inputcapacitors in this data sheet). The TPS series available fromAVX, and the 593D series from Sprague are both surge cur-rent tested.

To further simplify the buck regulator design procedure, Na-tional Semiconductor is making available computer designsoftware to be used with the Simple Switcher line ot switchingregulators. Switchers Made Simple (version 4.3 or later) isavailable on a 31⁄2" diskette for IBM compatible computers.

LM2596

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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

LM2596 Series Buck Regulator Design Procedure

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

OutputVoltage

(V)

Through Hole Output Capacitor Surface Mount Output Capacitor

Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward

HFQ Series Series Capacitor Series 595D Series Capacitor

(µF/V) (µF/V) (µF/V) (µF/V)

2 820/35 820/35 33 nF 330/6.3 470/4 33 nF

4 560/35 470/35 10 nF 330/6.3 390/6.3 10 nF

6 470/25 470/25 3.3 nF 220/10 330/10 3.3 nF

9 330/25 330/25 1.5 nF 100/16 180/16 1.5 nF

1 2 330/25 330/25 1 nF 100/16 180/16 1 nF

1 5 220/35 220/35 680 pF 68/20 120/20 680 pF

2 4 220/35 150/35 560 pF 33/25 33/25 220 pF

2 8 100/50 100/50 390 pF 10/35 15/50 220 pF

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

01258324

FIGURE 4. LM2596-3.3

01258325

FIGURE 5. LM2596-5.0

01258326

FIGURE 6. LM2596-12

01258327

FIGURE 7. LM2596-ADJ

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LM2596 Series Buck Regulator Design Procedure (Continued)

Inductance(µH)

Current(A)

Schott Renco Pulse Engineering Coilcraft

Through Surface Through Surface Through Surface Surface

Hole Mount Hole Mount Hole Mount Mount

L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223

L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683

L22 47 1.17 67144080 67144460 RL-5471-6 — PE-53822 PE-53822-S DO3316-473

L23 33 1.40 67144090 67144470 RL-5471-7 — PE-53823 PE-53823-S DO3316-333

L24 22 1.70 67148370 67148480 RL-1283-22-43 — PE-53824 PE-53825-S DO3316-223

L25 15 2.10 67148380 67148490 RL-1283-15-43 — PE-53825 PE-53824-S DO3316-153

L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S DO5022P-334

L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S DO5022P-224

L28 150 1.20 67144120 67144500 RL-5471-3 — PE-53828 PE-53828-S DO5022P-154

L29 100 1.47 67144130 67144510 RL-5471-4 — PE-53829 PE-53829-S DO5022P-104

L30 68 1.78 67144140 67144520 RL-5471-5 — PE-53830 PE-53830-S DO5022P-683

L31 47 2.20 67144150 67144530 RL-5471-6 — PE-53831 PE-53831-S DO5022P-473

L32 33 2.50 67144160 67144540 RL-5471-7 — PE-53932 PE-53932-S DO5022P-333

L33 22 3.10 67148390 67148500 RL-1283-22-43 — PE-53933 PE-53933-S DO5022P-223

L34 15 3.40 67148400 67148790 RL-1283-15-43 — PE-53934 PE-53934-S DO5022P-153

L35 220 1.70 67144170 — RL-5473-1 — PE-53935 PE-53935-S —

L36 150 2.10 67144180 — RL-5473-4 — PE-54036 PE-54036-S —

L37 100 2.50 67144190 — RL-5472-1 — PE-54037 PE-54037-S —

L38 68 3.10 67144200 — RL-5472-2 — PE-54038 PE-54038-S —

L39 47 3.50 67144210 — RL-5472-3 — PE-54039 PE-54039-S —

L40 33 3.50 67144220 67148290 RL-5472-4 — PE-54040 PE-54040-S —

L41 22 3.50 67144230 67148300 RL-5472-5 — PE-54041 PE-54041-S —

L42 150 2.70 67148410 — RL-5473-4 — PE-54042 PE-54042-S —

L43 100 3.40 67144240 — RL-5473-2 — PE-54043 —

L44 68 3.40 67144250 — RL-5473-3 — PE-54044 —

FIGURE 8. Inductor Manufacturers Part Numbers

Coilcraft Inc. Phone (800) 322-2645

FAX (708) 639-1469

Coilcraft Inc., Europe Phone +11 1236 730 595

FAX +44 1236 730 627

Pulse Engineering Inc. Phone (619) 674-8100

FAX (619) 674-8262

Pulse Engineering Inc., Phone +353 93 24 107

Europe FAX +353 93 24 459

Renco Electronics Inc. Phone (800) 645-5828

FAX (516) 586-5562

Schott Corp. Phone (612) 475-1173

FAX (612) 475-1786

FIGURE 9. Inductor Manufacturers Phone Numbers

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LM2596 Series Buck Regulator Design Procedure (Continued)

Nichicon Corp. Phone (708) 843-7500

FAX (708) 843-2798

Panasonic Phone (714) 373-7857

FAX (714) 373-7102

AVX Corp. Phone (803) 448-9411

FAX (803) 448-1943

Sprague/Vishay Phone (207) 324-4140

FAX (207) 324-7223

FIGURE 10. Capacitor Manufacturers Phone Numbers

VR 3A Diodes 4A–6A Diodes

Surface Mount Through Hole Surface Mount Through Hole

Schottky Ultra Fast Schottky Ultra Fast Schottky Ultra Fast Schottky Ultra Fast

Recovery Recovery Recovery Recovery

20V All ofthesediodes

arerated toat least

50V.

1N5820 All ofthesediodes

arerated toat least

50V.

All ofthesediodes

arerated toat least

50V.

SR502 All ofthesediodes

arerated toat least

50V.

SK32 SR302 1N5823

MBR320 SB520

30V 30WQ03 1N5821

SK33 MBR330 50WQ03 SR503

31DQ03 1N5824

1N5822 SB530

40V SK34 SR304 50WQ04 SR504

MBRS340 MBR340 1N5825

30WQ04 MURS320 31DQ04 MUR320 MURS620 SB540 MUR620

50V SK35 30WF10 SR305 50WF10 HER601

or MBRS360 MBR350 50WQ05 SB550

More 30WQ05 31DQ05 50SQ080

FIGURE 11. Diode Selection Table

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Application Information

PIN FUNCTIONS

+VIN — This is the positive input supply for the IC switchingregulator. A suitable input bypass capacitor must be presentat this pin to minimize voltage transients and to supply theswitching currents needed by the regulator.

Ground — Circuit ground.

Output — Internal switch. The voltage at this pin switchesbetween (+VIN − VSAT) and approximately −0.5V, with a dutycycle of approximately VOUT/VIN. To minimize coupling tosensitive circuitry, the PC board copper area connected tothis pin should be kept to a minimum.

Feedback — Senses the regulated output voltage to com-plete the feedback loop.

ON /OFF — Allows the switching regulator circuit to be shutdown using logic level signals thus dropping the total inputsupply current to approximately 80 µA. Pulling this pin belowa threshold voltage of approximately 1.3V turns the regulatoron, and pulling this pin above 1.3V (up to a maximum of 25V)shuts the regulator down. If this shutdown feature is notneeded, the ON /OFF pin can be wired to the ground pin orit can be left open, in either case the regulator will be in theON condition.

EXTERNAL COMPONENTS

INPUT CAPACITOR

CIN — A low ESR aluminum or tantalum bypass capacitor isneeded between the input pin and ground pin. It must belocated near the regulator using short leads. This capacitorprevents large voltage transients from appearing at the in-put, and provides the instantaneous current needed eachtime the switch turns on.

The important parameters for the Input capacitor are thevoltage rating and the RMS current rating. Because of the

relatively high RMS currents flowing in a buck regulator’sinput capacitor, this capacitor should be chosen for its RMScurrent rating rather than its capacitance or voltage ratings,although the capacitance value and voltage rating are di-rectly related to the RMS current rating.

The RMS current rating of a capacitor could be viewed as acapacitor’s power rating. The RMS current flowing throughthe capacitors internal ESR produces power which causesthe internal temperature of the capacitor to rise. The RMScurrent rating of a capacitor is determined by the amount ofcurrent required to raise the internal temperature approxi-mately 10˚C above an ambient temperature of 105˚C. Theability of the capacitor to dissipate this heat to the surround-ing air will determine the amount of current the capacitor cansafely sustain. Capacitors that are physically large and havea large surface area will typically have higher RMS currentratings. For a given capacitor value, a higher voltage elec-trolytic capacitor will be physically larger than a lower voltagecapacitor, and thus be able to dissipate more heat to thesurrounding air, and therefore will have a higher RMS cur-rent rating.

The consequences of operating an electrolytic capacitorabove the RMS current rating is a shortened operating life.The higher temperature speeds up the evaporation of thecapacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-facturers data sheet for maximum allowable RMS ripplecurrent. For a maximum ambient temperature of 40˚C, ageneral guideline would be to select a capacitor with a ripplecurrent rating of approximately 50% of the DC load current.For ambient temperatures up to 70˚C, a current rating of75% of the DC load current would be a good choice for aconservative design. The capacitor voltage rating must be atleast 1.25 times greater than the maximum input voltage,and often a much higher voltage capacitor is needed tosatisfy the RMS current requirements.

Block Diagram

01258321

FIGURE 12.

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Application Information (Continued)

A graph shown in Figure 13 shows the relationship betweenan electrolytic capacitor value, its voltage rating, and theRMS current it is rated for. These curves were obtained fromthe Nichicon “PL” series of low ESR, high reliability electro-lytic capacitors designed for switching regulator applications.Other capacitor manufacturers offer similar types of capaci-tors, but always check the capacitor data sheet.

“Standard” electrolytic capacitors typically have much higherESR numbers, lower RMS current ratings and typically havea shorter operating lifetime.

Because of their small size and excellent performance, sur-face mount solid tantalum capacitors are often used for inputbypassing, but several precautions must be observed. Asmall percentage of solid tantalum capacitors can short if theinrush current rating is exceeded. This can happen at turn onwhen the input voltage is suddenly applied, and of course,higher input voltages produce higher inrush currents. Sev-eral capacitor manufacturers do a 100% surge current test-ing on their products to minimize this potential problem. Ifhigh turn on currents are expected, it may be necessary tolimit this current by adding either some resistance or induc-tance before the tantalum capacitor, or select a higher volt-age capacitor. As with aluminum electrolytic capacitors, theRMS ripple current rating must be sized to the load current.

FEEDFORWARD CAPACITOR(Adjustable Output Voltage Version)

CFF — A Feedforward Capacitor CFF, shown across R2 inFigure 1 is used when the ouput voltage is greater than 10Vor when COUT has a very low ESR. This capacitor adds leadcompensation to the feedback loop and increases the phasemargin for better loop stability. For CFF selection, see thedesign procedure section.

OUTPUT CAPACITOR

COUT — An output capacitor is required to filter the outputand provide regulator loop stability. Low impedance or lowESR Electrolytic or solid tantalum capacitors designed forswitching regulator applications must be used. When select-ing an output capacitor, the important capacitor parametersare; the 100 kHz Equivalent Series Resistance (ESR), the

RMS ripple current rating, voltage rating, and capacitancevalue. For the output capacitor, the ESR value is the mostimportant parameter.

The output capacitor requires an ESR value that has anupper and lower limit. For low output ripple voltage, a lowESR value is needed. This value is determined by the maxi-mum allowable output ripple voltage, typically 1% to 2% ofthe output voltage. But if the selected capacitor’s ESR isextremely low, there is a possibility of an unstable feedbackloop, resulting in an oscillation at the output. Using thecapacitors listed in the tables, or similar types, will providedesign solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is re-quired, refer to the section on Output Voltage Ripple andTransients for a post ripple filter.

An aluminum electrolytic capacitor’s ESR value is related tothe capacitance value and its voltage rating. In most cases,higher voltage electrolytic capacitors have lower ESR values(see Figure 14 ). Often, capacitors with much higher voltageratings may be needed to provide the low ESR values re-quired for low output ripple voltage.

The output capacitor for many different switcher designsoften can be satisfied with only three or four different capaci-tor values and several different voltage ratings. See thequick design component selection tables in Figure 2 and 4for typical capacitor values, voltage ratings, and manufactur-ers capacitor types.

Electrolytic capacitors are not recommended for tempera-tures below −25˚C. The ESR rises dramatically at cold tem-peratures and typically rises 3X @ −25˚C and as much as10X at −40˚C. See curve shown in Figure 15.

Solid tantalum capacitors have a much better ESR spec forcold temperatures and are recommended for temperaturesbelow −25˚C.

CATCH DIODE

Buck regulators require a diode to provide a return path forthe inductor current when the switch turns off. This must bea fast diode and must be located close to the LM2596 usingshort leads and short printed circuit traces.

Because of their very fast switching speed and low forwardvoltage drop, Schottky diodes provide the best performance,especially in low output voltage applications (5V and lower).Ultra-fast recovery, or High-Efficiency rectifiers are also a

01258328

FIGURE 13. RMS Current Ratings for Low ESRElectrolytic Capacitors (Typical)

01258329

FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating(Typical Low ESR Electrolytic Capacitor)

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good choice, but some types with an abrupt turnoff charac-teristic may cause instability or EMI problems. Ultra-fastrecovery diodes typically have reverse recovery times of 50ns or less. Rectifiers such as the 1N5400 series are muchtoo slow and should not be used.

INDUCTOR SELECTION

All switching regulators have two basic modes of operation;continuous and discontinuous. The difference between thetwo types relates to the inductor current, whether it is flowingcontinuously, or if it drops to zero for a period of time in thenormal switching cycle. Each mode has distinctively differentoperating characteristics, which can affect the regulatorsperformance and requirements. Most switcher designs willoperate in the discontinuous mode when the load current islow.

The LM2596 (or any of the Simple Switcher family) can beused for both continuous or discontinuous modes of opera-tion.

In many cases the preferred mode of operation is the con-tinuous mode. It offers greater output power, lower peakswitch, inductor and diode currents, and can have loweroutput ripple voltage. But it does require larger inductorvalues to keep the inductor current flowing continuously,especially at low output load currents and/or high input volt-ages.

To simplify the inductor selection process, an inductor selec-tion guide (nomograph) was designed (see Figure 4 through8). This guide assumes that the regulator is operating in thecontinuous mode, and selects an inductor that will allow apeak-to-peak inductor ripple current to be a certain percent-age of the maximum design load current. This peak-to-peakinductor ripple current percentage is not fixed, but is allowedto change as different design load currents are selected.(See Figure 16.)

By allowing the percentage of inductor ripple current toincrease for low load currents, the inductor value and sizecan be kept relatively low.

When operating in the continuous mode, the inductor currentwaveform ranges from a triangular to a sawtooth type ofwaveform (depending on the input voltage), with the averagevalue of this current waveform equal to the DC output loadcurrent.

Inductors are available in different styles such as pot core,toroid, E-core, bobbin core, etc., as well as different corematerials, such as ferrites and powdered iron. The leastexpensive, the bobbin, rod or stick core, consists of wirewound on a ferrite bobbin. This type of construction makesfor an inexpensive inductor, but since the magnetic flux is notcompletely contained within the core, it generates moreElectro-Magnetic Interference (EMl). This magnetic flux caninduce voltages into nearby printed circuit traces, thus caus-ing problems with both the switching regulator operation andnearby sensitive circuitry, and can give incorrect scope read-ings because of induced voltages in the scope probe. Alsosee section on Open Core Inductors.

When multiple switching regulators are located on the samePC board, open core magnetics can cause interferencebetween two or more of the regulator circuits, especially athigh currents. A torroid or E-core inductor (closed magneticstructure) should be used in these situations.

The inductors listed in the selection chart include ferriteE-core construction for Schott, ferrite bobbin core for Rencoand Coilcraft, and powdered iron toroid for Pulse Engineer-ing.

Exceeding an inductor’s maximum current rating may causethe inductor to overheat because of the copper wire losses,or the core may saturate. If the inductor begins to saturate,the inductance decreases rapidly and the inductor begins tolook mainly resistive (the DC resistance of the winding). Thiscan cause the switch current to rise very rapidly and forcethe switch into a cycle-by-cycle current limit, thus reducingthe DC output load current. This can also result in overheat-ing of the inductor and/or the LM2596. Different inductortypes have different saturation characteristics, and thisshould be kept in mind when selecting an inductor.

The inductor manufacturer’s data sheets include current andenergy limits to avoid inductor saturation.

01258330

FIGURE 15. Capacitor ESR Change vs Temperature

01258331

FIGURE 16. (∆IIND) Peak-to-Peak InductorRipple Current (as a Percentage of the Load Current)

vs Load Current

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DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable forcontinuous mode operation, but for low current applicationsand/or high input voltages, a discontinuous mode designmay be a better choice. It would use an inductor that wouldbe physically smaller, and would need only one half to onethird the inductance value needed for a continuous modedesign. The peak switch and inductor currents will be higherin a discontinuous design, but at these low load currents (1Aand below), the maximum switch current will still be less thanthe switch current limit.

Discontinuous operation can have voltage waveforms thatare considerable different than a continuous design. Theoutput pin (switch) waveform can have some damped sinu-soidal ringing present. (See Typical Performance Character-istics photo titled Discontinuous Mode Switching Wave-forms) This ringing is normal for discontinuous operation,and is not caused by feedback loop instabilities. In discon-tinuous operation, there is a period of time where neither theswitch or the diode are conducting, and the inductor currenthas dropped to zero. During this time, a small amount ofenergy can circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ring-ing. Normally this ringing is not a problem, unless the ampli-tude becomes great enough to exceed the input voltage, andeven then, there is very little energy present to cause dam-age.

Different inductor types and/or core materials produce differ-ent amounts of this characteristic ringing. Ferrite core induc-tors have very little core loss and therefore produce the mostringing. The higher core loss of powdered iron inductorsproduce less ringing. If desired, a series RC could be placedin parallel with the inductor to dampen the ringing. Thecomputer aided design software Switchers Made Simple(version 4.3) will provide all component values for continu-ous and discontinuous modes of operation.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating inthe continuous mode will contain a sawtooth ripple voltage atthe switcher frequency, and may also contain short voltagespikes at the peaks of the sawtooth waveform.

The output ripple voltage is a function of the inductor saw-tooth ripple current and the ESR of the output capacitor. Atypical output ripple voltage can range from approximately0.5% to 3% of the output voltage. To obtain low ripple

voltage, the ESR of the output capacitor must be low, how-ever, caution must be exercised when using extremely lowESR capacitors because they can affect the loop stability,resulting in oscillation problems. If very low output ripplevoltage is needed (less than 20 mV), a post ripple filter isrecommended. (See Figure 1.) The inductance required istypically between 1 µH and 5 µH, with low DC resistance, tomaintain good load regulation. A low ESR output filter ca-pacitor is also required to assure good dynamic load re-sponse and ripple reduction. The ESR of this capacitor maybe as low as desired, because it is out of the regulatorfeedback loop. The photo shown in Figure 17 shows atypical output ripple voltage, with and without a post ripplefilter.

When observing output ripple with a scope, it is essentialthat a short, low inductance scope probe ground connectionbe used. Most scope probe manufacturers provide a specialprobe terminator which is soldered onto the regulator board,preferable at the output capacitor. This provides a very shortscope ground thus eliminating the problems associated withthe 3 inch ground lead normally provided with the probe, andprovides a much cleaner and more accurate picture of theripple voltage waveform.

The voltage spikes are caused by the fast switching action ofthe output switch and the diode, and the parasitic inductanceof the output filter capacitor, and its associated wiring. Tominimize these voltage spikes, the output capacitor shouldbe designed for switching regulator applications, and thelead lengths must be kept very short. Wiring inductance,stray capacitance, as well as the scope probe used to evalu-ate these transients, all contribute to the amplitude of thesespikes.

When a switching regulator is operating in the continuousmode, the inductor current waveform ranges from a triangu-lar to a sawtooth type of waveform (depending on the inputvoltage). For a given input and output voltage, thepeak-to-peak amplitude of this inductor current waveformremains constant. As the load current increases or de-creases, the entire sawtooth current waveform also risesand falls. The average value (or the center) of this currentwaveform is equal to the DC load current.

If the load current drops to a low enough level, the bottom ofthe sawtooth current waveform will reach zero, and theswitcher will smoothly change from a continuous to a discon-tinuous mode of operation. Most switcher designs (irregard-less how large the inductor value is) will be forced to rundiscontinuous if the output is lightly loaded. This is a per-fectly acceptable mode of operation.

01258332

FIGURE 17. Post Ripple Filter Waveform

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In a switching regulator design, knowing the value of thepeak-to-peak inductor ripple current (∆IIND) can be useful fordetermining a number of other circuit parameters. Param-eters such as, peak inductor or peak switch current, mini-mum load current before the circuit becomes discontinuous,output ripple voltage and output capacitor ESR can all becalculated from the peak-to-peak ∆IIND. When the inductornomographs shown in Figure 4 through 8 are used to selectan inductor value, the peak-to-peak inductor ripple currentcan immediately be determined. The curve shown in Figure18 shows the range of (∆IIND) that can be expected fordifferent load currents. The curve also shows how thepeak-to-peak inductor ripple current (∆IIND) changes as yougo from the lower border to the upper border (for a given loadcurrent) within an inductance region. The upper border rep-resents a higher input voltage, while the lower border repre-sents a lower input voltage (see Inductor Selection Guides).

These curves are only correct for continuous mode opera-tion, and only if the inductor selection guides are used toselect the inductor value

Consider the following example:

VOUT = 5V, maximum load current of 2.5A

VIN = 12V, nominal, varying between 10V and 16V.

The selection guide in Figure 5 shows that the vertical linefor a 2.5A load current, and the horizontal line for the 12Vinput voltage intersect approximately midway between theupper and lower borders of the 33 µH inductance region.A 33 µH inductor will allow a peak-to-peak inductor current(∆IIND) to flow that will be a percentage of the maximum loadcurrent. Referring to Figure 18, follow the 2.5A line approxi-mately midway into the inductance region, and read thepeak-to-peak inductor ripple current (∆IIND) on the left handaxis (approximately 620 mA p-p).

As the input voltage increases to 16V, it approaches theupper border of the inductance region, and the inductorripple current increases. Referring to the curve in Figure 18,it can be seen that for a load current of 2.5A, thepeak-to-peak inductor ripple current (∆IIND) is 620 mA with12V in, and can range from 740 mA at the upper border (16Vin) to 500 mA at the lower border (10V in).

Once the ∆IIND value is known, the following formulas can beused to calculate additional information about the switchingregulator circuit.

1. Peak Inductor or peak switch current

2. Minimum load current before the circuit becomes dis-continuous

3. Output Ripple Voltage = (∆IIND)x(ESR of COUT)

= 0.62Ax0.1Ω=62 mV p-p

4.

OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage orunstable operation is from an open core inductor. Ferritebobbin or stick inductors have magnetic lines of flux flowingthrough the air from one end of the bobbin to the other end.These magnetic lines of flux will induce a voltage into anywire or PC board copper trace that comes within the induc-tor’s magnetic field. The strength of the magnetic field, theorientation and location of the PC copper trace to the mag-netic field, and the distance between the copper trace andthe inductor, determine the amount of voltage generated inthe copper trace. Another way of looking at this inductivecoupling is to consider the PC board copper trace as oneturn of a transformer (secondary) with the inductor windingas the primary. Many millivolts can be generated in a coppertrace located near an open core inductor which can causestability problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor isused, it’s possible that the location of the inductor withrespect to other PC traces may be the problem. To deter-mine if this is the problem, temporarily raise the inductoraway from the board by several inches and then checkcircuit operation. If the circuit now operates correctly, thenthe magnetic flux from the open core inductor is causing theproblem. Substituting a closed core inductor such as a tor-roid or E-core will correct the problem, or re-arranging thePC layout may be necessary. Magnetic flux cutting the ICdevice ground trace, feedback trace, or the positive or nega-tive traces of the output capacitor should be minimized.

Sometimes, locating a trace directly beneath a bobbin in-ductor will provide good results, provided it is exactly in thecenter of the inductor (because the induced voltages cancelthemselves out), but if it is off center one direction or theother, then problems could arise. If flux problems arepresent, even the direction of the inductor winding can makea difference in some circuits.

This discussion on open core inductors is not to frighten theuser, but to alert the user on what kind of problems to watchout for when using them. Open core bobbin or “stick” induc-tors are an inexpensive, simple way of making a compactefficient inductor, and they are used by the millions in manydifferent applications.

01258333

FIGURE 18. Peak-to-Peak InductorRipple Current vs Load Current

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THERMAL CONSIDERATIONS

The LM2596 is available in two packages, a 5-pin TO-220(T) and a 5-pin surface mount TO-263 (S).

The TO-220 package needs a heat sink under most condi-tions. The size of the heatsink depends on the input voltage,the output voltage, the load current and the ambient tem-perature. The curves in Figure 19 show the LM2596T junc-tion temperature rises above ambient temperature for a 3Aload and different input and output voltages. The data forthese curves was taken with the LM2596T (TO-220 pack-age) operating as a buck switching regulator in an ambienttemperature of 25˚C (still air). These temperature rise num-bers are all approximate and there are many factors that canaffect these temperatures. Higher ambient temperatures re-quire more heat sinking.

The TO-263 surface mount package tab is designed to besoldered to the copper on a printed circuit board. The copperand the board are the heat sink for this package and theother heat producing components, such as the catch diodeand inductor. The PC board copper area that the package issoldered to should be at least 0.4 in2, and ideally shouldhave 2 or more square inches of 2 oz. (0.0028) in) copper.Additional copper area improves the thermal characteristics,but with copper areas greater than approximately 6 in2, onlysmall improvements in heat dissipation are realized. If fur-ther thermal improvements are needed, double sided, mul-tilayer PC board with large copper areas and/or airflow arerecommended.

The curves shown in Figure 20 show the LM2596S (TO-263package) junction temperature rise above ambient tempera-ture with a 2A load for various input and output voltages. Thisdata was taken with the circuit operating as a buck switchingregulator with all components mounted on a PC board tosimulate the junction temperature under actual operatingconditions. This curve can be used for a quick check for theapproximate junction temperature for various conditions, butbe aware that there are many factors that can affect thejunction temperature. When load currents higher than 2A areused, double sided or multilayer PC boards with large cop-per areas and/or airflow might be needed, especially for highambient temperatures and high output voltages.

For the best thermal performance, wide copper traces andgenerous amounts of printed circuit board copper should beused in the board layout. (One exception to this is the output(switch) pin, which should not have large areas of copper.)Large areas of copper provide the best transfer of heat(lower thermal resistance) to the surrounding air, and movingair lowers the thermal resistance even further.

Package thermal resistance and junction temperature risenumbers are all approximate, and there are many factorsthat will affect these numbers. Some of these factors includeboard size, shape, thickness, position, location, and evenboard temperature. Other factors are, trace width, totalprinted circuit copper area, copper thickness, single- ordouble-sided, multilayer board and the amount of solder onthe board. The effectiveness of the PC board to dissipateheat also depends on the size, quantity and spacing of othercomponents on the board, as well as whether the surround-ing air is still or moving. Furthermore, some of these com-ponents such as the catch diode will add heat to the PCboard and the heat can vary as the input voltage changes.For the inductor, depending on the physical size, type of core

material and the DC resistance, it could either act as a heatsink taking heat away from the board, or it could add heat tothe board.

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Circuit Data for Temperature Rise Curve

TO-220 Package (T)

Capacitors Through hole electrolytic

Inductor Through hole, Renco

Diode Through hole, 5A 40V, Schottky

PC board 3 square inches single sided 2 oz. copper(0.0028")

FIGURE 19. Junction Temperature Rise, TO-220

01258335

Circuit Data for Temperature Rise Curve

TO-263 Package (S)

Capacitors Surface mount tantalum, molded “D” size

Inductor Surface mount, Pulse Engineering, 68 µH

Diode Surface mount, 5A 40V, Schottky

PC board 9 square inches single sided 2 oz. copper(0.0028")

FIGURE 20. Junction Temperature Rise, TO-263

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DELAYED STARTUP

The circuit in Figure 21 uses the the ON /OFF pin to providea time delay between the time the input voltage is appliedand the time the output voltage comes up (only the circuitrypertaining to the delayed start up is shown). As the inputvoltage rises, the charging of capacitor C1 pulls the ON /OFFpin high, keeping the regulator off. Once the input voltagereaches its final value and the capacitor stops charging, andresistor R2 pulls the ON /OFF pin low, thus allowing thecircuit to start switching. Resistor R1 is included to limit themaximum voltage applied to the ON /OFF pin (maximum of25V), reduces power supply noise sensitivity, and also limitsthe capacitor, C1, discharge current. When high input ripplevoltage exists, avoid long delay time, because this ripple canbe coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where theinput power source is limited in the amount of current it candeliver. It allows the input voltage to rise to a higher voltagebefore the regulator starts operating. Buck regulators requireless input current at higher input voltages.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off untilthe input voltage reaches a predetermined voltage. An und-ervoltage lockout feature applied to a buck regulator isshown in Figure 22, while Figure 23 and 24 applies the samefeature to an inverting circuit. The circuit in Figure 23 fea-

tures a constant threshold voltage for turn on and turn off(zener voltage plus approximately one volt). If hysteresis isneeded, the circuit in Figure 24 has a turn ON voltage whichis different than the turn OFF voltage. The amount of hyster-esis is approximately equal to the value of the output volt-age. If zener voltages greater than 25V are used, an addi-tional 47 kΩ resistor is needed from the ON /OFF pin to theground pin to stay within the 25V maximum limit of the ON/OFF pin.

INVERTING REGULATOR

The circuit in Figure 25 converts a positive input voltage to anegative output voltage with a common ground. The circuitoperates by bootstrapping the regulator’s ground pin to thenegative output voltage, then grounding the feedback pin,the regulator senses the inverted output voltage and regu-lates it.

This example uses the LM2596-5.0 to generate a −5V out-put, but other output voltages are possible by selecting otheroutput voltage versions, including the adjustable version.Since this regulator topology can produce an output voltagethat is either greater than or less than the input voltage, themaximum output current greatly depends on both the inputand output voltage. The curve shown in Figure 26 provides aguide as to the amount of output load current possible for thedifferent input and output voltage conditions.

The maximum voltage appearing across the regulator is theabsolute sum of the input and output voltage, and this mustbe limited to a maximum of 40V. For example, when convert-ing +20V to −12V, the regulator would see 32V between theinput pin and ground pin. The LM2596 has a maximum inputvoltage spec of 40V.

Additional diodes are required in this regulator configuration.Diode D1 is used to isolate input voltage ripple or noise fromcoupling through the CIN capacitor to the output, under lightor no load conditions. Also, this diode isolation changes thetopology to closley resemble a buck configuration thus pro-viding good closed loop stability. A Schottky diode is recom-mended for low input voltages, (because of its lower voltagedrop) but for higher input voltages, a fast recovery diodecould be used.

Without diode D3, when the input voltage is first applied, thecharging current of CIN can pull the output positive by sev-eral volts for a short period of time. Adding D3 prevents theoutput from going positive by more than a diode voltage.

01258336

FIGURE 21. Delayed Startup

01258337

FIGURE 22. Undervoltage Lockoutfor Buck Regulator

01258338

This circuit has an ON/OFF threshold of approximately 13V.

FIGURE 23. Undervoltage Lockoutfor Inverting Regulator

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Because of differences in the operation of the invertingregulator, the standard design procedure is not used toselect the inductor value. In the majority of designs, a 33 µH,3.5A inductor is the best choice. Capacitor selection can also

be narrowed down to just a few values. Using the valuesshown in Figure 25 will provide good results in the majority ofinverting designs.

This type of inverting regulator can require relatively largeamounts of input current when starting up, even with lightloads. Input currents as high as the LM2596 current limit(approx 4.5A) are needed for at least 2 ms or more, until theoutput reaches its nominal output voltage. The actual timedepends on the output voltage and the size of the outputcapacitor. Input power sources that are current limited orsources that can not deliver these currents without gettingloaded down, may not work correctly. Because of the rela-tively high startup currents required by the inverting topology,the delayed startup feature (C1, R1 and R2) shown in Figure25 is recommended. By delaying the regulator startup, theinput capacitor is allowed to charge up to a higher voltagebefore the switcher begins operating. A portion of the highinput current needed for startup is now supplied by the inputcapacitor (CIN). For severe start up conditions, the inputcapacitor can be made much larger than normal.

01258339

This circuit has hysteresis

Regulator starts switching at VIN = 13V

Regulator stops switching at VIN = 8V

FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator

01258340

CIN — 68 µF/25V Tant. Sprague 595D

470 µF/50V Elec. Panasonic HFQ

COUT — 47 µF/20V Tant. Sprague 595D

220 µF/25V Elec. Panasonic HFQ

FIGURE 25. Inverting −5V Regulator with Delayed Startup

01258341

FIGURE 26. Inverting Regulator Typical Load Current

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INVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration issimple, pull it below 1.3V (@25˚C, referenced to ground) toturn regulator ON, pull it above 1.3V to shut the regulator

OFF. With the inverting configuration, some level shifting isrequired, because the ground pin of the regulator is nolonger at ground, but is now setting at the negative outputvoltage level. Two different shutdown methods for invertingregulators are shown in Figure 27 and 28.

01258342

FIGURE 27. Inverting Regulator Ground Referenced Shutdown

01258343

FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device

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TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED

01258344

CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

COUT — 330 µF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”


L1 — 47 µH, L39, Renco, Through Hole

Thermalloy Heat Sink #7020

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TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED

01258345

CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”

COUT — 220 µF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”


L1 — 47 µH, L39, Renco, Through Hole

R1 — 1 kΩ, 1%

R2 — Use formula in Design Procedure

CFF — See Figure 3.

Thermalloy Heat Sink #7020

FIGURE 29. PC Board Layout

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Physical Dimensions inches (millimeters)unless otherwise noted

5-Lead TO-220 (T)Order Number LM2596T-3.3, LM2596T-5.0,

LM2596T-12 or LM2596T-ADJNS Package Number T05D

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

5-Lead TO-263 Surface Mount Package (S)Order Number LM2596S-3.3, LM2596S-5.0,

LM2596S-12 or LM2596S-ADJNS Package Number TS5B

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORTDEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERALCOUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices orsystems which, (a) are intended for surgical implantinto the body, or (b) support or sustain life, andwhose failure to perform when properly used inaccordance with instructions for use provided in thelabeling, can be reasonably expected to result in asignificant injury to the user.

2. A critical component is any component of a lifesupport device or system whose failure to performcan be reasonably expected to cause the failure ofthe life support device or system, or to affect itssafety or effectiveness.

National SemiconductorCorporationAmericasEmail: [email protected]

National SemiconductorEurope

Fax: +49 (0) 180-530 85 86Email: [email protected]

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LM2596

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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

lm2596 simple switcher power converter 150 khz3a step …

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